DNA Testing: Introduction and Strategies

Intro to DNA Testing

DNA testing is a very useful supplement to traditional genealogical research. Things to remember:

DNA tests accurately identify other tested people who share one or more common ancestors with you

DNA tests don't identify how you're related to them

When both parties have access to a robust tree of their ancestors, the likely common ancestors can often be identified -
and help confirm the accuracy of their genealogical paper trails

There are three primary types of DNA tests. Here's how most people choose which one to take:

Autosomal (atDNA): the most popular and least expensive test. Available to men and women to discover information about ALL of their family lines.
Quite accurate back to four or so generations; can sometimes provide some useful information back to 7 or 8 generations.
Test as many people as you can afford, especially members of older generations (their results will be accurate further back in time.)

Y-Chromosome (y-DNA): available only to males (women can have a brother, uncle or male paternal cousin tested.) Tracks only the (one) paternal ancestral line,
but highly accurate back for 15 generations or more. The test is popular because people often have a special interest in the ancestors who carried their surname.
Usually only one person per extended family needs to be tested - as long as they all match via atDNA test.

Mitochondrial (mtDNA): available to men or women, but only tracks the maternal line (mother to mother's mother to mother's grandmother...). Like
the y-DNA test, this can track one ancestral line (but maternal) far into the past.
Because surnames generally change in each generation, it's often difficult to determine how you're related to any matches.
This is the least popular of the tests.

atDNA

Autosomal DNA is inherited equally from both parents - half from the father and half from the mother.

The corollary to this is that we each do NOT inherit half of our parents' atDNA.

Similarly, each sibling inherits half of each parent's atDNA - but it will be a random mix of the half that you inherit and the half that you do not.
On average, each sibling will inherit half of the half that you inherit from each parent, and half of the half that you do not (but this can vary significantly).

Since all of your atDNA is inherited from your parents, most people who are able to test both parents don't need to test themselves or their siblings.
(There are some advanced techniques like phasing and chromosome mapping where it's useful to have a child's results as well.)

If you can't test both parents, test as many siblings as possible.
On average, one sibling will be missing 50% of each parent's atDNA, two siblings (between them) will only be missing 25% of each parent's atDNA,
three siblings will be missing 12.5%, and so on.

Identical twins have identical atDNA.
Fraternal twins carry the same randomly overlapping (and non-overlapping) atDNA amounts as compared to any other two siblings.

The amount of autosomal DNA inherited from more distant ancestors is randomly shuffled up in a process called recombination,
and the percentage of autosomal DNA coming from each ancestor is diluted with each new generation.
Every individual receives an average of 25% of their atDNA from each of their four grandparents,
but this often ranges from 20 to 30% from each (always adding to 100%, of course.)

Always test a parent if possible.
They carry 50% of each of their parent's atDNA, while you may carry as low as 20% of each of their parents - and very rarely more than 30%.

While you may carry some atDNA that a (paternal, for example) aunt or uncle doesn't carry,
they will always carry some atDNA that your father doesn't - and (very close to) always carry more of your paternal atDNA than you do.

atDNA Inheritance Diagram

The diagram above compares the amount of atDNA people share on average with various relatives. A few points:

Everyone has four grandparents, eight great-grandparents, sixteen great-great-grandparents, and so on.

On average, each individual inherits 6.25% of their atDNA from each great-great-grandparent, but the actual number varies significantly.

Full third-cousins share two of their sixteen great-great-grandparents.
The third cousins each receive about 6.25% of their atDNA from each of the two shared g-g-grandparents - but it's not the same 6.25%.

The resulting amount of atDNA each third cousin actually shares ends up averaging about 0.781%.

atDNA Testing Strategies

This leads to several atDNA testing strategies:

If both of your parents are living, test them immediately.
There is usually then no need to test yourself or your siblings, as all of your atDNA comes from one parent or the other.

Test as many members of the oldest living generation as possible when one or both of their parents haven't been tested.
Your father's sister, for example, probably inherited a very significant amount of their father's atDNA that your father did not.

Assuming that cost is a consideration, try to test at least one cousin in each family's oldest generation - and one second cousin if possible.
They will often carry a significant amount of atDNA that your grandparents (or great-grandparents) didn't inherit.

y-DNA

Each of us has a 23rd pair of chromosomes that are inherited differently than the 22 pairs of autochromosomes.
Men have one X and one Y chromosome in the 23rd pair while women have two X chromosomes.

"A Y chromosome DNA test (Y-DNA test) is a genealogical DNA test which is used to explore a man's patrilineal or direct father's-line ancestry.
The Y chromosome, like the patrilineal surname, passes down virtually unchanged from father to son.
Every now and then occasional mistakes in the copying process occur,
and these mutations can be used to estimate the time frame in which the two individuals share a most recent common ancestor or MRCA. "
- Source: International Society of Genetic Genealogy

Females never inherit a Y chromosome from their father; their mother doesn't have one.

Males inherit a Y chromosome that is passed down virtually unchanged from father to son for generations.

Males and females both inherit an X chromosome from their mother.

Females inherit a second X chromosome from their father.

No paternal ancestors make any contribution to the X chromosome.
Even though we all have 128 ancestors at the 7th generation (half of which are male), only 21 of them (all female) contribute to the X chromosome.

y-DNA Tests

Different levels of y-DNA testing provide greater degrees of accuracy. Markers are specific places on the y-DNA whose contents are examined during testing
to see what it contains and assigns
a value